It has long been known that the specific power output of internal combustion engines can be increased by supplying pressurized air to the engine inlet port such that a larger charge of air can be inducted into the combustion chamber or chambers of the engine during each cycle. In a spark ignition engine it is a fuel/air mixture that is induced, whereas in a compression ignition engine it is air that is induced, with fuel being injected directly into the compressed charge.
The application of forced induction to spark ignition engine presents various problems, one of which is that of pre-ignition of the air/fuel charge prior to the normal ignition timing. Such pre-ignition (or knocking) degrades engine performance and applies damaging shock forces to the engine; it must be avoided during normal engine operation. The necessity to avoid pre-ignition limits the compression ratio that can be utilized in spark ignition engines, it may require the use of high octane fuels which delay the inception of pre-ignition, and it has limited the circumstances under which forced induction can be used effectively, as well as necessitating the use of intercoolers to cool the compressed air before it is applied to the engine. In typical applications, this has entailed that forced induction can be applied effectively only under essentially full throttle conditions at high engine speeds. Under such conditions it is undoubtedly effective in increasing specific power output, but in most automotive applications, high speed full throttle operation occurs for only a very small proportion of the total operating time of the engine. It has been necessary to avoid conditions under which excessive temperatures and pressures occur with the combustion chamber(s) of the engine during the compression stroke since these tend to result in pre-ignition. One approach to this problem has involved so-called lean-burn technology, as disclosed for example in U.S. Pat. No. 5,135,235 (Yanagihara). In such an engine system, the boost applied by the forced induction system is also related to the air/fuel ratio of the mixture applied to the engine, with this ratio increasing with increasing boost of the intake air. This tends to reduce the tendency to pre-ignition, while the increased volume of air passed through the system assists somewhat in reducing engine temperatures, and somewhat extends the range over which some boost can be effectively applied.